Anatomy of a Black Hole

By definition a black hole is a region where matter collapses to infinite density, and where, as a result, the curvature of spacetime is extreme. Moreover, the intense gravitational field of the black hole prevents any light or other electromagnetic radiation from escaping. But where lies the "point of no return" at which any matter or energy is doomed to disappear from the visible universe?

Applying the Einstein Field Equations to collapsing stars, German astrophysicist Kurt Schwarzschild deduced the critical radius for a given mass at which matter would collapse into an infinitely dense state known as a singularity. For a black hole whose mass equals 10 suns, this radius is about 30 kilometers or 19 miles, which translates into a critical circumference of 189 kilometers or 118 miles.

If you envision the simplest three-dimensional geometry for a black hole,
that is a sphere (known as a Schwarzschild black hole), the black hole's
surface is known as the event horizon. Behind this horizon, the inward
pull of gravity is overwhelming and no information about the black hole's
interior can escape to the outer universe.

Apparent versus Event Horizon

As a doomed star reaches its critical circumference, an "apparent"
event horizon forms suddenly. Why "apparent?" Because it separates light rays that are trapped inside a black hole from those that can move away from it. However, some light rays that are moving away at a given instant of time may find themselves trapped later if more matter or energy falls into the black hole, increasing its gravitational pull. The event horizon is traced out by "critical" light rays that will never escape or fall in.

Even before the star meets its final doom, the event horizon forms at the center, balloons out and breaks through the star's surface at the very moment it shrinks through the critical circumference. At this point in time, the apparent and event horizons merge as one: the horizon. For more details, see the caption for the above diagram.

The distinction between apparent horizon and event horizon may seem subtle, even obscure. Nevertheless the difference becomes important in computer simulations of how black
holes form and evolve.

Beyond the event horizon, nothing, not even light, can escape. So the event horizon acts as a kind of "surface" or "skin" beyond which we can venture but cannot see. Imagine what happens as you approach the horizon, then cross the threshold.

At the center of a black hole lies the singularity, where matter is crushed to infinite density, the pull of gravity is infinitely strong, and spacetime has infinite curvature. Here it's no longer meaningful to speak of space and time, much less spacetime. Jumbled up at the singularity, space and time cease to exist as we know them.

The Limits of Physical Law

Newton and Einstein may have looked at the universe very differently, but they would have agreed on one thing: all physical laws are inherently bound up with a coherent fabric of space and time.

At the singularity, though, the laws of physics, including General Relativity, break down. Enter the strange world of quantum gravity. In this bizzare realm in which space and time are broken apart, cause and effect cannot be unraveled. Even today, there is no satisfactory theory
for what happens at and beyond the singularity.

Cosmic Censorship

It's no surprise that throughout his life Einstein rejected the possibility of singularities. So disturbing were the implications that, by the late 1960s, physicists conjectured that the universe forbade "naked singularities." After all, if a singularity were "naked," it could alter the whole universe unpredictably. All singularities within the universe must therefore be "clothed."

But inside what? The event horizon, of course! Cosmic censorship is thus enforced. Not so, however, for that ultimate cosmic singularity that gave rise to the Big Bang.

Science versus Speculation

We can't see beyond the event horizon. At the singularity, randomness reigns supreme. What, then, can we really "know" about black holes? How can we probe their secrets? The answer in part lies in understanding their evolution right after they form.